The relationship between density fractionated human red cells and their in vivo age will be studied. Centrifugal density fractionation of human red cells in their own cohort labelled normal red cell populations will be fractionated at various periods through their life span. The density fractions will be characterized in order to establish the adequacy of the fractionation technique for preparing cells of different ages as well as the most appropriate indicator tests of red cell age (e.g., enzyme activity, NANA released, etc.). Methods will be refined for the enzymatic release and subsequent analysis of total membrane-bound sialic acid. In vitro phagocytosis will be evaluated as an indicator of in vivo red cell elimination. The observation that dense ("old") human red cells are phagocytosed to a greater extent than the least dense ("young") red cells by autologous monocytes in vitro may provide a means of evaluating certain aspects of red cell elimination in vivo. There is as yet insufficient evidence to establish the validity of this test system. As a consequence we plan to commence examination of the validity of this test system during the 59Fe labelling studies in humans and subsequently during erythrocyte life span studies in rabbits.